1. Field of the Invention
The invention relates to a device to monitor corrosion and other structural changes in steel reinforced structures, such as concrete bridges, roadways and load bearing support members. This instrument is fully embeddable and becomes a permanent component of the structure.
2. Brief Description of the Prior Art
Corrosion monitoring has been a recognized problem and has been addressed by the prior art. Unfortunately, the prior art has a number of weak points that render the more exacting readings impossible. Further, the prior art technologies have concentrated on instruments which are specifically designed to be embedded in steel reinforced concrete structures and monitor electrochemical corrosion related parameters and cannot be used for other applications.
The use of Ag/AgCl reference electrodes present a long-term stability issue that is overcome in the disclosed device through the use of MnO2 reference electrodes, thereby enabling the instrument to produce accurate and repeatable measurements over its useful lifetime.
The prior art corrosion monitoring devices embedded the electrode containing probe in the concrete with a cable connecting the electrode to the electronic instrumentation and power source at the exterior of the structure. Although this makes the electronic instrumentation and power source accessible, this system adversely affects the accuracy of the readings by making the accuracy inversely proportional to the length of the cable between the electrodes and the analog signal processing electronics.
This inaccuracy problem was overcome in the disclosed system through the inclusion of the electronic sensors within the instruments. By placing the electronic sensors within the instruments, the signal loss is dramatically reduced, enabling far more accurate readings.
The use of integrating type A/D converter integrated circuits with 16-bit resolution within the prior art devices has also created accuracy problems. The accuracy of an integrating type A/D converter is largely dependant upon the performance of external components such as the converter system""s integrating capacitor. The capacitance value of external integrating capacitors can vary significantly with changes in ambient humidity and temperature, making them sub-optimal for embedded instrument applications in any material that is subject to being affected by ambient changes. Also, these converters do not have the ability to perform on-chip amplification and filtering which is important when processing analog signals having a low signal to noise ratio. The disclosed device has overcome this problem through the use of a high-resolution 24-bit sigma/delta type A/D converter with programmable gain amplifier and low pass filter.
An additional problem encountered by prior art devices are that they are only capable of communication with one other device and are not capable of networked multi-point communications. They further do not have the capability to keep track of time or have on-board data-logging capability. The disclosed devices are capable of digital communication with one another as well as with a single external data collection instrument. The networking, in combination with the xe2x80x9creal time clockxe2x80x9d enables controlled scheduling of tests, reporting of data, etc.
The need to improve the methods of monitoring structural materials has been recognized. In 1998-1999 research was done and articles published regarding the advantages of measuring corrosion electrochemically. Embedded Sensor for Corrosion Measurement, SPIE Vol. 3587-0277-786X/99, R. G. Kelly, J. Yuan, S. H. Jones, W. Wang, K. Hudson, A. Sime, O. Schneider, G. G. Clemena;
One method of solving the problem of embedded electrodes was to place everything on a chip using on chip or off chip electrodes. The problem with this method is that when on chip electrodes are used, there is insufficient surface to provide accurate readings. The incorporation of outboard electrodes present the same problem as prior art devices with an inability to transmit a strong enough signal over the distance between the electrode and the electronics. This technology only used a potentiostat having a simple signal in/signal out capability. Embeddable Microinstruments for Corrosion Monitoring, R. G. Kelly, J. Yuan, S. H. Jones, W. Blanke, J. H. Aylor, W. Wang, A. P. Batson, Paper 97294, 1997.
In An ASIC for Electrochemical Measurement of Corrosivity in Concrete, J. Yuan, W. Wang, S. H. Jones, A. Wintenberg, R. G. Kelly uses a chip with a potentiostat and a galvanostat that rely on off board electrodes and separate processor. This again continues the prior art problem of losing signal due to the transmission distance. The Corrosion Monitoring in Concrete by Embeddable Microinstruments; R. G. Kelly, J. Yuan, S. H. Jones, J. H. Aylor, W. Wang, A. B. Batson, A. Wintenberg, G. G. Clemena again uses an arrangement similar to that of the foregoing monitoring instrument, without overcoming the loss of signal problem. Embeddable Microinstruments for Corrosivity Monitoring in Concrete, R. G. Kelly, S. H. Jones, O. M. Schneider, W. Wei, J. Yuan, A. Sime illustrates a Power Point presentation of the technology using the ASIC and, obviously continues to have the same limitations as the original presentation.
The prior art trend was toward extreme miniaturization through the design of an ASIC (application specific integrated circuit). This miniaturization presented the problem that the circuits performance wasn""t sufficiently high to monitor and transmit repeatable and accurate signals. Additionally, the expense involved in the design and development of an ASIC is so high that the addition of varied, or multiple monitoring, brings the costs above practicality.
In addition to the resolution of the loss of signal, none of the articles address the environmentally specific issues associated with protecting the embedded electronics and electrodes from mechanical damage in a potentially rugged material. Nor was the issue of how to accurately read one or more instruments addressed. The prior art did not, however, address the networking and data sharing issues that are required for the accurate coverage of large structures such as bridges, multi-story buildings, etc. Not only is the networking and data sharing of instruments far more complicated than connecting two instruments via a point-to-point connection, the signal must be accurately transmittable.
These and other problems have been overcome by the disclosed invention to produce an embeddable monitoring device where the electrodes, sensor electronics and microprocessor are all contained in a ruggedized and moisture tight case. The proximity of the electrodes, sensor electronics and microprocessor enables the transmission of maximum signal, more accurate readings and networkability.
A system for monitoring the material changes in a structure is disclosed through the use of at least one monitoring instrument embedded within the structure. For large structures, the instruments can be networked to provide readings from each specific portion of the structure. The monitoring instruments have at least one sensor with each sensor having electrodes in contact with the surrounding material. Electronics for each electrode are contained within the instrument and receive analog signals from the electrodes. An analog to digital converter converts the signals from each of the sensor prior to the signals being sent to a microcontroller. The analog to digital converter also amplifies and filters the signals and, in one embodiment, the amplification and filtration are programmable from the data logger. A digital to analog converter converts signals being sent to the electronic sensors from the microcontroller. A transmission device transmits the digital signals from the microcontroller to an external data logger or computer for display of the digitized signals. The connection between the microcontroller and the data logger can be either through hardwire or radio frequency (RF). A real time clock, in two-way communication with the microcontroller, can also be incorporated. Power is provided to the electronic sensors through either external or local methods. To conserve power, a power management system can be used that is in communication with each of the electronics sensors, the analog to digital converter, microcontroller, and digital to analog converter. The power management system regulates the power consumption by placing any of the electronic sensors, analog to digital converter, microcontroller, and digital to analog converter into a sleep mode when not in use.
When the system is used to monitor corrosion a galvanostat, or equivalent, is used to measure conductivity and a potentiostat or its equivalent used to measure linear polarization resistance. Chloride concentration is measured through the use of an Ag/AgCl electrode that is Clxe2x88x92 ion specific, and a MnO2 reference electrode by measuring the voltage potential between the ion specific electrode and the MnO2. The device can alternatively use a steel working electrode and a stainless steel reference electrode to measure the linear polarization of the area surrounding the instrument by matching the exterior surface of the steel working electrode to the exterior surface of the surrounding support steel.
When using local at least one of a piezoelectric generator; an electrochemical galvanic couple; and/or a RF power receiver using an impedance matching network is used. Preferably a combination of the piezoelectric generator, electrochemical galvanic couple and RF power receiver are used and serve as an alternate power source to one another.
To protect the electronic sensors and enable them to be embedded into the structural material an instrument case is used that is manufactured from a material having a flexural modulus at least equal to said structural material stress divided to by said structural material strain. The flexural modulus equal to, or greater than, the concrete preventing mechanical failure of the device before failure of the concrete. The instrument case has a hollow body with a removable first end, a closed second end and multiple sides, each of the multiple sides having a connection length with an adjacent side. The body is configured to contain and protect the electronic sensors from contact by with the structural material. The removable first end has at least one electrode receiving port and, in the preferred embodiment, at least one protective tray. The protective trays extending from the face at about a right angle to surround the electrode receiving ports, thereby enabling the electrodes to be raised from the surface of the first end. The instrument has at least one cable port to receive a network cable for connection to other instruments and/or a power source. The adjacent connection lengths are rounded to direct pressure from the surrounding structural material around the instrument case to prevent cracks from forming due to the pressure asserted by the surrounding material. The first end of the instrument case contains receiving ports to receive the external electrodes of the electrical sensors, placing a first end of each electrode in contact with said structural material and a second end each electrode in electronic contact with the electronics within the instrument case. The instrument case preferably has attachment flanges integral to the hollow body with tie receiving channels to enable the instrument case to be attached to the surrounding steel structure. The modules are protected within the instrument case by potting material preferably having sufficient flexibility to permit the enclosure to flex without compressing against the electronic components.